1. Field of the Invention
The present invention relates to a location measurement technique of a mobile terminal, and more specifically to a reference signal transmission method and system for measuring the location of a mobile terminal using a Time Difference of Arrival (TDOA) scheme etc., a location measurement method, device, and system using the same, and a time synchronization method and device using the same.
2. Description of the Related Art
Technologies for measuring the location of a Mobile Terminal (MT) include a network-based scheme such as an Angle of Arrival (AOA), Time of Arrival (TOA) and TDOA, a handset-based scheme such as a Global Positioning System (GPS), and a hybrid scheme combining a network-based scheme and a handset-based scheme.
A GPS scheme uses a satellite signal provided from a GPS satellite and thus can offer an accurate location of a Mobile Terminal (MT). However, many signal blocking areas are present and the MT needs to be equipped with a GPS receiver in order to implement the GPS scheme.
A location measurement technology that utilizes TDOA or TOA is used, and it is mainly applied in a shadow area of GPS when the GPS receiver is not provided or a hybrid GPS is used.
A location measurement technology using the TDOA scheme will now be briefly described. A time difference of arrival of a signal, TDOA, which is proportional to the difference between distances of a receiver (an MT for example) from two signal sources (two Base Stations (BSs) for example) is measured, and the MT is located on any one of points at which the difference between distances of the MT from the two BSs is constant, i.e. on a hyperbola, the focuses of which are the two BSs. Through this process, two hyperbolas are obtained from three BSs and an intersection point of the two hyperbolas is estimated as the location of the MT.
A location measurement technology using the TOA scheme will now be briefly described. A time of arrival of a signal, TOA, between an MT and a BS is measured to measure a distance therebetween and the MT is located on a circle, the center of which is the BS and the radius of which is the measured distance. Three circles are obtained from at least three BSs and an intersection point of the three circles is estimated as the location of the MT.
To acquire a TDOA between each BS and an MT in the TDOA scheme or a TOA between each BS and an MT in the TOA scheme, time synchronization between each BS and an MT is necessary. This will be described in more detail with reference to FIGS. 1 and 2.
FIG. 1 is a view referred to for describing a TDOA calculation method in a conventional TDOA scheme in order to confirm the location of an MT.
Referring to FIG. 1, two BSs, i.e. a BS A and a BS B, transmit signals synchronized at an absolute time. An MT receives a signal delayed by a signal shift time T2 between the BS A and the MT from the BS A and receives a signal delayed by a signal shift time T3 between the BS B and the MT from the BS B.
Using the signals received by the MT, TOAs T2 and T3 from the BSs to the MT can be obtained or a TDOA T4 (=T3−T2) from the two BSs to the MT can be obtained.
FIG. 2 is a view referred to for describing a TDOA calculation method in a conventional TDOA scheme or a TOA calculation method in a conventional TOA scheme in order to confirm the location of an MT.
Referring to FIG. 2, an MT transmits a predetermined signal (including MT unique information etc.) synchronized at an absolute time to a BS A and a BS B.
The BSs A and B receive a signal transmitted by the MT based on an absolute time. That is, the BS A receives a signal delayed by a signal shift time T2 between the BS A and the MT from the MT and the BS B receives a signal delayed by a signal shift time T3 between the BS B and the MT from the MT. A TOA T2 of the signal from the MT to the BS A can be obtained using the signal received by the BS A, and a TOA T3 of the signal from the MT to the BS B from the MT can be obtained using a signal received by the BS B. The difference between the TOAs, i.e. a TDOA T4 (=T3−T2), of the signals from the MT to the two BSs can also be obtained.
In this case, time synchronization between each BS and an MT, which is indispensable in the location measurement technology of the aforementioned TDOA or TOA scheme, is performed using an additional time synchronization system such as a satellite clock.
The time synchronization system plays a major role in achieving a TDOA or TOA based location measurement system. Accordingly, the presence of the time synchronization system causes a substantial increase in the size and cost of the location measurement system.
Furthermore, it is very difficult to achieve a time synchronization system capable of accurately synchronizing each BS. Errors occurring in the process of time synchronization have a negative effect upon the location measurement of the MT.